While many RPG franchises fill themselves to the brim with lore, there aren't many that go to the same level as Kingdom Hearts. Almost every installment in the series adds to its overarching story in some way, including the now-offline mobile game Kingdom Hearts Union X. While it was questionable how much a game starring the player as its protagonist could leave an impact on Sora's story, Union X ended up leaving an important mark.
Union X is anything but short. While stories in RPG franchises are often on the long side, Union X in particular featured 979 story quests by the end of the game's run. Between this length and the mobile nature of the game, many fans are likely put off from finishing it. Kingdom Hearts has been leaning into Union X as a big part of its lore since KH3, and the game offers poignant foreshadowing to the series' next arc.
Kingdom Hearts: The Story of the Keyblade War
Many Kingdom Hearts fans should be familiar with the Keyblade War. Union X dives into what led up to the war and reveals it was set to bring an end to the fight between light and darkness once and for all. Darkness itself was said to be almost human once, splitting itself into pieces and losing its will and humanity as it became the Heartless many fans are more familiar with.
In order to stop the pieces of darkness, the Master of Masters gave six apprentices roles, secretly knowing that the roles would cause them to doubt one another and grow headstrong in their ways, making them susceptible - yet incorruptible - by darkness. The Keyblade War served to draw out darkness, trap it, and destroy it. However, darkness worked around this and saved itself through the Master of Master's own contingency plan.
The True Story of Union X
Foreteller Ava's role was to create the Dandelions, a group of exceptional Keyblade wielders who would go on to live in a Data World and re-create other worlds with the Lux, or light, they've gathered throughout playing Kingdom Hearts Union X. This digital world would be led by new Union leaders chosen by the Master of Masters: Ephemer, Skuld, Brain, Lauriam, and Strelitzia. These Union leaders become recurring characters in the series as a whole.
However, Strelitzia was taken out by the darkness as a way to escape into the Data World that was meant to protect the light. The darkness possessed another Keyblade wielder, Ventus, so he could act as a Trojan Horse. This ended up revealing that Ventus was in possession of a heart of pure light, and he absorbed the darkness as part of himself permanently to save his friends. This darkness would later reform as Vanitas from Birth by Sleep.
Union X Leads into Kingdom Hearts' Future
Lauriam, Strelitzia's brother, ends up investigating her death with the help of another Dandelion, Elrena. The Union leaders find out they've been lied to by the Foretellers about the true nature of the Data World, and that an intruding Maleficent triggered an emergency self-destruct feature. Brain works to making sure they all get out safely.
All the Union leaders make it out safely, but other Keyblade wielders - such as the player character - weren't so lucky. The Dandelions who didn't make it out of the Data World get put to sleep as Dream Eaters, implied to be the very same ones Sora meets in Kingdom Hearts: Dream Drop Distance. The player's character in particular decides to remain, reforming into a new heart. The dreams that Xehanort had as a child in Dark Road imply the Union X players become Xehanort in the end.
Using the same Lifeboats from Kingdom Hearts: Melody of Memory to escape into different parts of time, Ephemer escapes to build Scala Ad Caelum above the ruins of Daybreak Town; his spirit as a Union leader helps Sora at the Keyblade Graveyard in Kingdom Hearts 3. Lauriam and Elrena appear in different Disney worlds, but the two meet again as the nobodies Marluxia and Larxene in Organization 13 later on. Ventus is found at the Keyblade Graveyard, unconscious after absorbing Darkness, and is made an apprentice under Xehanort.
Strelitzia is eventually found at the Final World in Melody of Memory, offering a very important hint to Kairi and Riku. Sora is most likely in a sort of "fictional" world, a world beyond what humans can comprehend, as described by the Master of Masters. In one of the final cutscenes for Union X, the Master tells the darkness a solution to their feud can only be found in a world that light and dark can't reach, with the implication that it could be Quadratum.
Kingdom Hearts Union X is available as an offline game on Android and iOS devices.